One of the things I like so much about the earlier material published for the World of Greyhawk is that so much was left to the DM. That will sound like a contradiction to those who are more used to the modern style, where everything is detailed and no bit of minutiae is left undisclosed. But looking back on the original Folio edition, and the first Boxed Set, the DM was given only the barest amount of information necessary to get going, and then was left to fill in the rest on his or her own.
Compare this with the Forgotten Realms, where everything was given to the DM through innumerable guides, books, articles, and so forth. Ed Greenwood’s material was encyclopedic, and he is to be commended for the amount of detail he brought to his own campaign world, but when it came to being a published game setting, very little was really left to the DM to do, other than run players through adventures. And woe betide the DM who didn’t keep up with the deluge of information, as small details, once overlooked, could come back to bite you later on when they became pivotal. And you had no idea which ones were the important bits! All of those books and supplements told you everything you could possibly want to know about the Realms.
That’s not to say that Greyhawk didn’t have details. It most certainly did. But even in presenting its details (and I am here speaking of the pre-Wars material), the World of Greyhawk did things differently. There was never any “Depths of the Oerth” supplement, dutifully cataloging the various races, places, and “adventure hooks” to be found in the warrens of tunnels beneath the Hellfurnaces. Nowhere was there a “Complete Guide to Giants” with details of their social structure, sample encounter areas, relations between the tribes of giants, etc. No! What you got were adventure modules. You didn’t find out about the Depths of Oerth by reading about them; you found out about it by running your characters through the modules that described it. And the details were sparse indeed; the DM was expected to take the information in the module and riff off of it as a starting point. There was never a map of Erelhei-Cinlu for a reason, I am convinced. It was left for each DM to make it his or her own. All those adventure modules showed you what the necessary details were, through example, and the DM was left to fill in the vast gaps in detail.
Other settings told you everything you could possibly want to know. Greyhawk showed you what you absolutely needed to know, and left you to fill in the rest.
It’s the empty room principle writ large; give the enterprising DM the barest outline, and he will step up to the challenge. It’s shame that newer versions of the game have entirely dropped this principle, but it is understandable. There’s a lot of money to be made in producing endless supplements, books, and so forth.